Due to the trends toward weight reduction high performance of automobiles, springs have been strengthened and a high-strength steel having a tensile strength exceeding 1,500 MPa, after heat treatment, has been applied to springs. In recent years, a steel wire having a tensile strength exceeding 1,900 MPa has also been required. The purpose is to secure a material hardness which does not cause problems when the material is used as a spring even though the material softens to some extent by heating in stress relief annealing, nitriding and the like when manufacturing a spring.
As a means to secure such a material, Japanese Unexamined Patent Publication No. S57-32353 discloses a method of generating fine carbides, which dissolve during quenching and precipitate during tempering, by adding elements such as V, Nb, Mo, etc., and by so doing, controlling the movement of dislocations and thus improving setting resistance.
In the meantime, as methods to produce a steel coil spring, there are the hot-coiling method wherein a steel is heated to a temperature in an austenite region, coiled, and then quenched and tempered, and the cold-coiling method wherein a high-strength steel wire prepared by subjecting a steel to quenching and tempering beforehand is coiled in a cold state. In the cold-coiling method, since oil tempering treatment, high-frequency treatment or the like capable of employing rapid heating and rapid cooling when producing a steel wire can be used, it is possible to reduce the prior austenite grain size of a spring material and, as a result, a spring excellent in fracture property can be produced. Further, the method has an advantage of reducing the equipment cost for a spring maker since an installation such as a heating furnace in a spring manufacturing line can be simplified, and therefore a shift to the cold-coiling of a spring has advanced in recent years.
However, when the strength of a steel wire for a cold-coiled spring increases, it happens frequently that the steel wire breaks during the cold-coiling and cannot be formed into the shape of a spring. Therefore, there has been no other way than to coil a steel wire by a method which cannot provide strength and workability simultaneously and seems to be industrially disadvantageous. Usually, in the case of a valve spring, a steel wire after being subjected to quenching and tempering, namely oil tempering, on-line. is coiled. For example, in Japanese Unexamined Patent Publication No. H05-179348, a wire is heated and coiled at a temperature where the wire is easily transformed during coiling to prevent breakage during the coiling in such a manner that a wire is heated to a temperature of 900 to 1,050° C. and coiled, and after that is tempered at a temperature of 425 to 550° C., and thereafter the wire is subjected to conditioning treatment after the coiling to secure high strength. Such heating during coiling and conditioning after the coiling cause the dispersion of spring dimensions after heat treatment or the radical deterioration of treatment efficiency, and therefore a spring produced by this method is inferior to a cold-coiled spring in both the cost and the dimensional accuracy.
With regard to the grain size of carbides, an invention developed by noticing the average grain size of V or Nb system carbides is disclosed in Japanese Unexamined Patent Publication No. H10-251804, for example, and the invention shows that with only the control of the average grain size of V or Nb carbides, sufficient strength and toughness cannot be obtained. Moreover, in this prior art, it is stated that there is a concern that an abnormal structure appears, which is caused by cooling water during rolling, and, therefore, it is recommended to substantially employ dry rolling. From this description, it is assumed that the art involves unsteady work industrially and is apparently different from usual rolling, and it suggests that even though the average grain size is controlled, troubles in rolling occur when the unevenness of a nearby matrix structure is generated.